We upgraded from Exchange 5.5 to GroupWise 6 about 6 months ago, and have found it to be excellent. The limitations that were suggested earlier (attachment size, mailbox size, and restore) are not really issues. Attachment size can be set in the GWIA, mailbox size can be set in GW6 (not in 5.x, they only allowed size by days), and the restore is ok if you use ArcServe on a NON-NSS volume with the GroupWise agent. If you don't want to go for the agent, you can restore easily by restoring just the user.db file, the msgxx.db and a pointer to Offiles directory (a little tricky, but only necessary if you need attachments).
Anyway, good solid package. Great wireless features in WebAccess. WebAccess in general is the best of the three big products (GW, EX, Notes).
One big reason we went to GW was cost and it doesn't seem to get nailed with viruses.
Not completely unbiased, but we have used several different packages in the last few years, and I think this one will stick.